The perceived vulnerability of Sen. Pat Toomey in the general election, in which the Republican presidential nominee may be more liability than asset, has produced three competitive Democratic candidates to challenge the incumbent.

JOE SESTAK, a former Navy admiral who served two terms in the House, has the best credentials and experience to immediately serve Pennsylvania and the nation. His unrelenting intellectual curiosity has helped Sestak, a notoriously hard taskmaster, form a deep understanding of government and foreign policy that Katie McGinty and John Fetterman cannot match.

Sestak, 64, makes the most persuasive argument that he can maneuver beyond partisan politics to help the middle class rebound from years of stagnant wages. Like his opponents, he wants to close tax loopholes that encourage companies to send jobs overseas, but his plans are more detailed. He also wants to fine-tune the Affordable Care Act to lessen its impact on small businesses.

With trade agreements taking up so much of the conversation among candidates for the presidency on down, Sestak refuses to get caught up in hyperbole and offers a more nuanced approach to correcting trade imbalances. He believes American jobs must be protected but rules out protectionist policies that could hurt alliances and cost jobs by closing foreign markets.

McGinty, 52, served as a high-ranking environmental official under both President Bill Clinton and Gov. Ed Rendell. With female voters expected to make a big difference in this year's elections, party officials as high as President Obama have endorsed her candidacy. But her campaign has had trouble making the case that she is ready for the Senate.

Fetterman, 46, the hulk-size, tattooed mayor of Braddock, a neglected steel town in Allegheny County, brings extraordinary authenticity to the race. The graduate of Harvard and the University of Connecticut has lived amid the poverty and crime that most politicians only talk about, but Fetterman's inability to do more than highlight the problems he wants to solve disqualifies him for such a high political post.

As a congressman, Sestak sponsored or cosponsored close to 20 bills that passed. The father of a child diagnosed with brain cancer was a founder of the Pediatric Cancer Caucus. He successfully sponsored alternative-energy legislation. And his aggressiveness is credited with saving nearly 1,000 families in his Delaware County district from losing homes to foreclosures.

Sestak commanded carrier battle groups in Afghanistan and Iraq, was the Navy's chief planning officer at the Pentagon, directed defense policy for Clinton, and developed antiterrorism strategy. In each of those jobs, he exhibited the dogged determination and attention to detail that would make him a valuable contributor to foreign-policy debates.

Sestak's refusal to be put into a partisan box may have contributed to his inability to get the endorsement of some Democratic heavyweights. But demonstrating his willingness to put principle above party may improve Sestak's chances in a second contest against Toomey, who beat him by only two percentage points in 2010.